In X-ray tube anodes, only a fraction of the supplied electrical energy is converted into X-ray radiation energy. Most of the energy is converted into undesirable heat, which subjects the anodes to severe temperature stresses.
In the past, there has been no lack of attempts to devise ways to enable rapid removal of the thermal energy produced in X-ray anodes, primarily by seeking to increase thermal emissivity at the anode surface. A known way for increasing the thermal emissivity of X-ray anodes is to deposit oxide coatings onto the anode surface. The oxide coatings typically contain a certain proportion of titanium dioxide, which results in a blackening effect. These oxide top layers are also frequently fused, after their application to the anode, by a thermal treatment, which results in a still improved thermal emission factor and an improved adhesion of the coating layer to the substrate material.
EP-A2 0 172 491 describes an X-ray anode made of a molybdenum alloy, such as the molybdenum alloy TZM, having an oxide coating composed of a mixture of 40-70% titanium dioxide, the remainder being composed of stabilized oxides from the group comprising ZrO.sub.2, HfO, MgO, CeO.sub.2, La.sub.2 O.sub.3 and SrO. This prior disclosure describes fusing the oxide coating in order to improve both the thermal emission coefficient and the adhesion of the oxide layer to the parent body. The disadvantage of such an X-ray anode is that the carbon contained in the parent body of the rotary anode brings about a severe ageing of the oxidic top layer, which leads to a premature deterioration of the thermal emission coefficient.
Austrian Patent Specification 376 064 described an X-ray tube rotary anode having a parent body of a carbon-containing molybdenum alloy, for example TZM, which is provided, outside of the focal track region, with a surface coating for improving thermal emissivity that is composed of one or more oxides or of a mixture composed of one or more metals with one or more oxides to improve the thermal emissivity. This prior disclosure proposes arranging a 10-200 .mu.m thick interlayer made of molybdenum and/or tungsten between the parent body and the oxide coating, in order to prevent the rapid ageing of the rotary anode and thus the premature reduction of the thermal emission coefficient. A disadvantage of such a rotary anode is that fused oxide coatings virtually can not be produced. It has been found that, depending on the manner of deposition of the molybdenum and/or tungsten interlayer, the oxidic top layer cannot be caused to fuse at all, or it runs off the surface to be coated during fusion.